Rice blast, which is caused by the fungus Magnaporthe grisea, is one of the most devastating diseases in rice, and occurs in most rice growing areas worldwide. In terms of plant damage, rice blast commonly causes leaf blast during the vegetative phase of rice plant development, and infertility when plants are infected during the reproductive phase (panicle and node blast). This latter effect can result in dramatic yield and quality reductions, which are estimated to result in economic losses for farmers of nearly $5 billion per year (Moffat (1994) Science 265:1804-1805).
Because rice farmers generally have limited economic resources, control of rice blast is most often accomplished through the use of rice plant cultivars that exhibit a natural resistance to the disease. However, the disease resistance exhibited by these cultivars is generally unstable, with cultivars released as resistant showing susceptibility after only a few seasons of widespread cultivation. Despite this instability, the use of resistant cultivars remains the most economical and effective method of controlling rice blast disease. Consequently, there is a continued need for such disease-resistant cultivars.
Although resistant cultivars occur naturally, recent research has focused on genetic methods for creating or improving highly disease-resistant plants. Thus for the last four decades, rice geneticists and breeders have studied the genetics of blast resistance germplasm in order to develop the methods necessary to breed such durably resistant cultivars. Methods for the genetic analysis of resistance to blast originated in the early 1960s when Goto established the differential system for races of M. grisea in Japan (Ou (1985) Rice Disease 2nd ed. (Commonwealth Mycological Institute, Slough, UK).
One blast resistance gene of particular importance is the Pi2 gene, which exhibits highly effective broad-spectrum resistance to a diverse population of blast disease isolates and, consequently, remains effective in a wide range of rice cultivation areas after over a decade of use. Although the location of this gene in the rice genome has been determined (Yu et al. (1991) Theor. Appl. Genet. 81:471-476; Liu et al. (2002) Mol. Genet. Genom. 267:472-480), its DNA sequence remains unknown. Because techniques for creating or improving disease resistance rely on the knowledge of such sequences, there is a great need for obtaining the actual DNA sequence of the Pi2 gene.